Human behavior can be a spectacle to say the least. Not too far from animals (take the feline below), psychologists have attempted to explain how humans behave and how we become the people we are. Ultimately, regardless of perspective, this leads to growth and change.
Psychologists have explained human development through sexuality, cognition (thoughts), and social. Freud made his Psychosexual theory of development(5 phases). Piaget's Cognitive theory of development looked at development in 4 phases. Erik Erickson chose to see the process from a social perspective, which involved stages that ran into a person's middle years.
However, I believe human behavior cannot be summed up with one theory. It cannot be given boundaries with ages through phases.
The growth that a human experiences is a process. I like to think of Dr Carl Rogers' book entitled On Becoming a Person: a Therapist's View on Psychotherapy. As with becoming a person, the same can be said of an aspect in a person's life. When there exists an area needing attention, it takes time to work on that area. It's a process that doesn't end.
Consider the individual who is shy. This impacts his relationships. He does not have the easiest time talking to women. He struggles with speaking in front of people. Fortunately, he has the insight to know this needs to be worked on.
Eventually, he works up the courage to join a public speaking group. Overtime, he manages to go up to a random woman and talk to her (see the male below putting the moves on the blonde). Just talk. Then, he gets the coveted seven-digit number.
man talking to a woman
These baby steps pay dividends, but he is not finished. He has not reached the finish line.
This process explains a lot of phenomena and is like an infinite marathon, the longest of marathons without an ending. You can pace yourself in this marathon though. This is the plight of a human, the plight of human growth.